Electric motor designs often require that the stator have large-area slots (where the width of the slot is greater than the radial length, or depth, of the slot). A conventional slotted stator without heat spikes is shown in FIG. 4. The stator structure includes a cylindrical outer shell 40 made of laminated silicon steel. This shell provides the outer back iron for the motor. The stator shown has six teeth 42 and therefore it has six slots 44. The six stator teeth 42 are equally spaced around the circumference of the stator shell 40. The stator teeth 42 extend radially inward from the stator shell Since the stator teeth 42 are equally spaced they define six slots 44 of approximately equal area. Within the stator slots, electrical conductive wires, known as stator windings, carry current. These stator windings are not perfect conductors and, therefore, the current traveling through them generates heat within the slots.
The heat generated by the stator windings causes the temperature of the motor's constituent materials to rise. As the temperature of the constituent materials rise, the properties of those materials degrade. For example, the energy product of the magnets used decreases as the temperature increases. Thereby, the generated heat causes a decrease in the motor's continuous load rating. Furthermore, the materials used to insulate the windings can break down at higher temperatures and the life of the insulating materials will decrease.
The decreased performance is more pronounced in motors with large-area slots. The generated heat dissipates along a path from the windings to the lamination iron and motor housing. Thus, the heat generated nearer to the slot's center does not dissipate as easily as that nearer to its boundary since the heat path is longer. Accordingly, the temperature near the slot center rises above the average temperature of the windings The greater the distance that the slot center is from the slot boundary, that is, the larger the slot area, the greater this temperature difference (called the hot spot temperature) becomes This hot spot temperature limits the motor's continuous load rating because the current in the windings must be limited so that the insulation temperature rating is not exceeded.
Despite the above-mentioned problems, electric motors are still designed with large area slots for various reasons, including the reduced manufacturing cost due to the small number of stator slots. However, for the motors with large area slots there is a need to improve the continuous load rating by improving the heat transfer from the stator windings to the motor housing.